Mysterious Megatunnels in South America Were Dug by Giant Sloths

These huge tunnels scattered around South America have long puzzled scientists over their origins. Huge and unnaturally smooth, the mysterious caves are no product of any known natural geologic process, especially when large claw marks were discovered on the ceilings and walls of the tunnels.

The answer? It may sound strange, but these giant tunnels are indeed the works of giant sloths. To be specific, now-extinct species of giant ground sloths painstakingly built these massive structures at least 8,000 to 10,000 years ago, according to a comprehensive report from Discover Magazine.

After stumbling on his first tunnel -- now dubbed "paleoburrows" -- Brazilian geologist Heinrich Frank and his team uncovered at least 1,500 just in Frank's home state of Rio Grande do Sul in south Brazil. Further north in Santa Catarina, there are hundreds more and counting.

"There's no geological process in the world that produces long tunnels with a circular or elliptical cross-section, which branch and rise and fall, with claw marks on the walls," Frank explained. "I've [also] seen dozens of caves that have inorganic origins, and in these cases, it's very clear that digging animals had no role in their creation."

Scientists have known of these paleoburrows for decades, but their origins remained a mystery or assumed to be man-made.

In 2010, a few years after Frank first began studying the paleoburrows, another Brazilian geologist Amilcar Adamy sought out a strange cave in Rondonia in the Amazon, one that exhibited the same features.

Upon returning for a more comprehensive investigation in 2015, Adamy found out that this Amazon paleoburrow is one of the largest ever discovered. The different branches of the tunnels added up to about 2,000 feet long with the main shafts believed to be originally six feet tall and three to five feet wide.

"This wasn't made by one or two individuals," says Adamy. "It was made by many, over generations."

Frank and his team believes that ground sloths were responsible for the biggest burrows. Several genera were suggested, particularly due to their fossils that seemed to suggest a propensity for digging: Catonyx, Glossotherium and Lestodon. Other researchers have suggested extinct armadillos as alternatives.